When a person sinks to the bottom of a pool of water, or even floats unconscious on the surface of the water, his lungs stop functioning and his brain is therefore deprived of oxygen. If this situation is allowed to continue for more than a few minutes, irreparable damage will be done to the brain even if the person is resuscitated. However, if the drowning person is detected quickly and removed from the water, there is still a very short time period during which resuscitation may be effected before permanent damage ensues. Thus, rapid detection and identification of a drowning body is essential if resuscitation is to be at all possible and if permanent damage is to be avoided.
Clearly, detection of a drowning body which remains floating on the surface of the water may easily be effected manually by means of a life-guard, when present. Consequently, effort has generally been expended into detecting a drowning body which is submerged beneath the surface of the water. Most prior art devices are based on detecting disturbances in the water and actuating a suitable alarm when such disturbances are detected. Such an approach may also be used in order to monitor unauthorized entry into a swimming pool and to provide an alarm in the event of such unauthorized entry.
Thus, for example, U.S. Pat. No. 4,747,085 (Dunegan) discloses a swimming pool alarm system for actuating an alarm in response to the movement of a person in a swimming pool. The system includes a transmitter mounted below the surface of the swimming pool for continuously transmitting ultrasonic sound waves through the body of water, and a similarly positioned receiver detects sound waves in the water and generates an electrical signal in response thereto. The electrical signal is continuously monitored, and the movement of a person in the pool alters the received ultrasonic sound waves and the corresponding electrical signal so as to produce a corresponding alarm signal.
The device disclosed by Dunegan et at., and which is typical of its genre, is based on the assumption that a drowning person struggles violently prior to sinking and therefore gives rise to disturbances in the water which can be monitored.
In contrast to such an approach, U.S. Pat. No. 5,043,705 (Rooz et al.) discloses a system for detecting a motionless body in a pool consequent to a swimmer sinking to the bottom of the pool, usually after losing consciousness. In such circumstances, the swimmer no longer moves and therefore detection based on monitoring disturbances in the water is no longer possible. The system disclosed by Rooz et al. is based on scanning the lower surface of the pool so as to extract successive frames of image data which are analyzed in order to detect a contour of a body which remains substantially motionless for longer than a predetermined time interval.
There are drawbacks associated with both of the above-mentioned approaches. Thus, devices of the type described by Dunegan et al. are intended to provide a warning in the event of unauthorized disturbances within the swimming pool. Such systems are effective only when the pool is empty. People swimming in the pool inevitably cause disturbances which require that the alarm systems of the type described by Dunegan et al. be disabled in order to prevent them from operating. In other words, alarm systems of this type are primarily intended to protect domestic swimming pools or even public swimming pools when not in use and the life-guard is not present.
The purpose of such alarm systems is generally to protect against unforeseen slipping in the water and to provide a warning that someone has entered the pool when it is, as yet, unauthorized for use. Such an approach is obviously important to guard against small children slipping into the water or against somebody who falls in when it is dark and cannot climb out unaided.
The fact that such devices must be disabled when use of the pool is authorized, severely militates against their use for detecting a swimmer in distress. Such a drawback is overcome by the system disclosed by Rooz et al., but their system is sophisticated and therefore expensive.
Yet another drawback associated with all systems of the type described is that they must somehow be mounted in conjunction with the swimming pool itself. Thus, specifically, there are provided ultrasonic transmitters and detectors which must be affixed to the pool. This might possibly be acceptable for domestic pools, but provides insufficient protection for a member of the public who may swim regularly at different pools. Unless each pool is protected by a suitable monitoring device which is enabled when the pool is open to the public, the resulting protection is inadequate.
Such a drawback would, of course, be overcome if the detection means typically associated with prior art systems were somehow associated not with the pool but rather with the swimmer himself. No such approach has been contemplated in hitherto proposed systems.